The present invention pertains to a means for monitoring the lower thread on double thread lockstitch sewing machines with a preferably rotating hook.
A means for recognizing the approaching end of the lower thread of a sewing machine with a double thread lockstitch hook, which makes it possible to still have a defined residual thread length available after the recognition of the approaching thread end regardless of the particular thread strength being used, is shown in DE 40 31 058 C1.
A web, which is directed essentially in parallel to the flanges of the bobbin and whose lateral surfaces are reflective, is provided for this purpose on the hub of the bobbin. Openings are provided in the flanges of the bobbin, whose surfaces have a light-absorbing layer or have a dull finish, for the infrared rays emitted from a reflection head, which fall laterally on the thread roll, passing through the flange of the bobbin or through the openings in the flange when the bobbin is still filled sufficiently, and are absorbed. As long as the infrared rays are not reflected by the reflection head, no signal can be sent by the reflection head.
If the thread roll has been used up to the extent that the external diameter of the residual thread length is smaller than the external diameter of the web, the infrared rays passing through the openings in the flange fall on the lateral surface of the web and are sent back to the reflection head. A signal, which is used for the further control of the sewing machine and which can be used either to stop the machine immediately or to still form a defined number of residual stitches, is generated as a result.
Since it makes no difference for the absorption of the infrared ray whether it falls on the stationary surface of the flange of the bobbin or on the stationary thread roll or whether the latter are rotating, the approaching end of the bobbin thread can be recognized or displayed with this means in the case of a machine operating properly, but it is not possible to recognize a disturbance in sewing, e.g., a thread break, because if a break of the needle thread or of the lower thread occurs during sewing while the bobbin is still filled with a relatively large amount of thread, the infrared ray is not sent back by the flange of the bobbin or the thread roll to the reflection head, and no output signal is consequently generated.
An improvement of the above-described means, which pertains essentially to the increase in the reliability of operation of the said means, is shown in DE 41 15 520 A1, in which infrared rays emitted by a reflection head are likewise returned by a reflection surface arranged at the bottom of the hook body to the reflection head. Openings for the infrared ray are provided for this purpose both in the upper part of the bobbin housing, on the one hand, and in the two flanges of the bobbin, on the other hand. Moreover, the bobbin is equipped in an advantageous embodiment with two webs arranged at spaced locations from one another, in which openings, which are arranged on the same pitch circle as the openings in the flanges of the bobbin, are likewise provided for the infrared ray. The arrangement of all openings is selected now to be such that in the case of a bobbin having only a residual thread length, the infrared ray emitted by the reflection head reaches the reflection surface through the openings of the upper part of the bobbin housing, of the first bobbin flange, the two webs, the lower part of the bobbin housing as well as the hook body and is returned therefrom to the reflection head. The signal originating from this as a result can then be used either to stop the machine immediately or to still work off at least part of the residual thread length.
If the bobbin is still filled with thread to such an extent that the openings in the webs of the bobbin are covered by the residual thread roll, the infrared ray is absorbed. It makes absolutely no difference whether this happens with the bobbing rotating or stopped. If breaking of the needle thread or lower thread occurs during sewing with the bobbin still filled with a relatively large amount of thread, the infrared ray is not returned by the flange of the bobbin or the thread roll to the reflection head in this means, either, and no output signal is consequently generated, either.
Even though contamination of the reflection surface by settling dust generated during the sewing is counteracted in this means by the needle thread moving over the reflection surface during the stitch formation, this means is nevertheless relatively susceptible to fault because of the plurality of openings through which the infrared light ray passes and is correspondingly expensive to manufacture, especially because it additionally also requires an adapted bobbin housing lower part and a hook body provided with a reflection foil besides an adapted bobbin housing top part and a corresponding bobbin.
DE 34 47 138 A1 shows a means for double thread lockstitch sewing machines, which generates signal sequences that can be assigned to the particular instantaneous operating state depending on that operating state, wherein a signal sequence with a sequence changing in a pronounced sinus-like pattern is generated in the case of a sufficiently filled bobbin and trouble-free sewing operation, a signal sequence with intermittent residual ripples is generated in the case of a bobbin that is no longer filled sufficiently, and, finally, a signal sequence of a constant pattern is generated in the case of stoppage of the bobbin, which corresponds to a disturbance. When the bobbin is still filled sufficiently and rotating, the infrared light ray emitted by a reflection head is reflected alternatingly by the thread reserve covering the openings and by the reflecting surfaces of the particular bobbin flange, which surfaces are located between these openings, as a result of which the above-mentioned signal sequence changing in a pronounced sinus-like pattern is generated. By contrast, the openings are free in the case of a bobbin having only the residual thread length, and the signal sequence is formed alternatingly by the likewise reflecting inner side of a first bobbin flange and the reflecting outer side of the second bobbin flange. Even though this signal sequence is called a xe2x80x9ca signal sequence with horizontal residual ripplesxe2x80x9d in A1, with respect to FIG. 3 in that document, it can be assumed in light of the essentially equal degree of reflection of the inner and outer sides of the bobbin flanges that a signal sequence thus formed is basically a continuous signal. Since the infrared light ray reaches the inner side of a bobbin flange either through one of the openings and is reflected by same or falls on the area of the outer side of the bobbin flange located between the openings and is reflected by same during the stoppage of the bobbin, it is obvious that there can be no substantial difference between the signal sequence generated in the case of a bobbin that is filled only with a residual thread length and is rotating and the bobbin that is likewise filled only with a residual thread length but is stopped. It is therefore extremely difficult to distinguish a rotating bobbin from a stopped bobbin based on the signal sequences thus generated in the case of bobbins filled with a residual thread length only.
The basic object of the present invention is therefore to provide a means for monitoring the three possible operating states of a double thread lockstitch sewing machine, namely, when the degree of filling of the bobbin is still above a residual thread length, or the machine is processing thread of the residual thread length, or when there is a disturbance causing the stoppage of the bobbin, e.g., a thread break, which means can be manufactured at a low cost and is very extensively insensitive to contamination by dust generated during sewing.
According to the invention, a monitor is provided for a bobbin thread on double thread lockstitch sewing machines. This is preferably a sewing machine with a rotating hook with a bobbin housing accommodating the lower thread reserve. The monitor includes a revolution sensor which sends a pulse per revolution of the main shaft of the machine to a microcomputer. An infrared laser light transmitter sends a detection beam directed toward a bobbin. The beam is reflected by the bobbin or by the roll of a residual thread length to a receiver and is sent by the latter to the microcomputer in the form of signals for processing, evaluation and/or forwarding. The microcomputer is adapted to compare the different signal patterns with a detection threshold (DS) to generate signals for controlling the sewing machine. A controllable power source is connected to the microcomputer for the occasional, stepwise reduction of the output of the infrared laser light transmitter. A bobbin can be inserted into the bobbin housing with at least two chambers separated from one another by a radially directed web. One chamber (the residual thread chamber) accommodates a residual thread length and the other chamber (the main chamber) accommodates the main thread length. The front surface of the web faces the infrared laser light transmitter as well as the outer surface of the flange of the bobbin. This outer surface is adjacent to the front surface, reflecting light, wherein the flange has openings for the detection beam.
The use of an infrared laser light transmitter offers the advantage of coherent radiation, such as a high degree of parallelism and consequently intense bundling of the radiation, high emission output and high monochromatism compared with the infrared light transmitters used hitherto.
Especially because of the high parallelism and the high emission output, the advantage is achieved here that both the infrared laser light transmitter and a receiving phototransistor acting as a receiver can be arranged at a relatively great distance from the bobbin and deflecting mirrors or prisms can thus be eliminated, on the one hand, and the susceptibility to dust is substantially reduced, on the other hand.
Furthermore, bobbins with a degree of reflection usual for metals can be used in the means according to the present invention, so that it is also possible to use, e.g., commercially available bobbins with a chamber for the residual thread length and with a chamber for the main thread length, and the distance between the outer side of the flange of the bobbin and the front surface of the web limiting the chamber for the residual thread length, which said front surface is located adjacent to the outer side, is sufficient for obtaining signals of sufficient intensity from the flange of the bobbin and the front surface of the web.
If the microcomputer has a stitch countdown counter, into which the number of stitches or seams or workpieces that can be prepared with the residual thread length can be entered before the beginning of processing of the residual thread length, and which can be counted down by a revolution sensor of the sewing machine. The microprocessor generates a signal for the control unit of the sewing machine for stopping the machine and/or activates a display means after the formation of the preset number of stitches or seams or workpieces. The monitor according to the present invention is especially suitable for use in group stitch sewing machines or automated sewing units.
To make it possible to recognize a stoppage of the bobbin caused by, e.g., thread break with the infrared laser beam falling on the roll of the residual thread length, the monitor has an additional stitch countdown counter, which can be set to a preselectable number of stitches each time the detection threshold is exceeded by the signal derived from the reflected detection beam and can be counted down by pulses sent by the revolution sensor to the microcomputer, in order to trigger a signal for stopping the machine when the zero value is reached. The sewing machine can thus be stopped a few stitches after the stoppage of the bobbin.
To achieve the highest possible insensitivity of the means according to the present invention to the effects caused by the dust generated during sewing, the infrared laser light transmitter is operated with a current intensity that comes close to its capacity. Even though this has the advantage of a high reliability in operation, it is also associated with the drawback that regardless of whether the detection beam falls on the flange of the bobbin or on the front side of the web, a continuous reflection is generated, whose nearly constant signal does not make it possible to obtain information on whether the continuous reflection can be attributed to the processing of the residual thread length or to the stoppage of the bobbin. To filter out the sinus-like signal formed during the processing of thread of the residual thread length, which changes over from one side of the detection threshold to the other side of the detection threshold, from the constant signal generated by the stoppage of the bobbin, the microcomputer has a stitch counter, which can be started by a signal change from below to above the detection threshold in order to generate a signal for the stepwise reduction of the current that can be sent to the infrared laser light transmitter by the power source from its normal operating intensity to a minimum intensity in the case of the absence of a signal change after a predeterminable number of stitches.
In order not to also carry out the stepwise reduction of the current to be sent to the infrared laser light transmitter to its minimum intensity when a signal change is already recognized during this and it is thus determined that the continuous reflection is caused during the processing of thread of the residual thread length, this reduction can be terminated by the recognized signal change.
If the second flange of the bobbin also has openings for the detection beam, it makes no difference with which of the flanges the bobbin is first introduced into the bobbin housing, i.e., it makes no difference for the scanning which of the two flanges is located adjacent to the infrared laser light transmitter. If the web is now arranged, as is shown in the drawing, eccentrically to the two flanges and the bobbin is introduced into the bobbin housing such that the flange located adjacent to the web is located adjacent to the infrared laser light transmitter, the conditions that are shown in the exemplary embodiment and also described will arise for both the residual thread chambers and the scanning of the bobbin.
If, by contrast, it is ensured during the winding up of the thread that, relative to FIG. 1, the inner area of the main chamber 9 is filled with thread first, then the residual thread chamber and subsequently the remaining area of the main chamber 9 and the bobbin thus filled is then introduced into the bobbin housing such that the inner area of the main chamber is located adjacent to the infrared laser light transmitter, a larger volume is obtained for the now broader residual thread chamber, i.e., a sufficient residual thread length can be introduced even in the case of a thicker thread. Since the distance between the reflection surfaces of the flange and of the web leads to different intensities between the detection beam emitted by the infrared laser light transmitter and the reflected detection beam sent to the receiver, a reliable evaluation result is obtained in this case as well.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.